The New Almanac

Basil

Ocimum basilicum

A tender, warm-season herb that frost kills outright — wait for warm nights, pinch it often, and keep water off the leaves to fend off downy mildew.

At a glance

Sun
Full sun / Partial shade
Water
Moderate
Soil pH
6–7.5
Spacing
6–12 in
6–12 in apart (UMN/Utah State); Clemson spaces larger types up to 2 ft
Germination soil
70–85°F

Mature size: Size depends heavily on the variety — compact types stay around 8–10 in, while Genovese and other sweet basils can reach 18–24 in or more.

Hardiness

Frost-tender and grown as a warm-season annual across the US. It collapses at the first frost, so its season runs only from settled warm weather to the first fall freeze.

Sun

Six to eight hours of bright light a day (UMN); Clemson allows full sun to part sun with about 6 hours.

Water

About 1½ inches of water per week, applied at the base — wet foliage invites downy mildew.

Planting calendar

Timed to your own frost dates — look up yours by ZIP on the home page.

  • Start indoors 4–8 weeks before your last spring frost. Clemson suggests 4–6 weeks; UMN allows up to 6–8 weeks before setting out.
  • Transplant — Set out only after all danger of frost has passed, once days are consistently in the 70s°F and nights stay above 50°F. Transplant seedlings that have 2–3 pairs of true leaves.
  • Direct sow — Sow after the last frost, once the soil has warmed.

Basil is a tender warm-season herb that wants the same conditions as a tomato: heat, sun, and settled weather. Cold soil stunts it and frost kills it outright, so patience in spring pays off — a basil plant set out too early sulks while a later one races past it.

Wait for warm nights

Don't rush basil into the ground. Wait until days are consistently in the 70s°F and nights stay reliably above 50°F. Chilled basil develops blackened, pitted leaves and never fully recovers, so a week or two of extra patience produces a far better plant.

Pinch for a bushier plant

Harvest by cutting whole stem tips down to just above a pair of leaves once the plant has 6–8 leaves, leaving 2–4 leaves behind so it branches and regrows. Pinch off flower buds as they form — letting basil flower and set seed signals the plant to wind down. (Pinching prevents that decline rather than directly forcing new growth.)

Guard against downy mildew

Downy mildew is basil's most common and most destructive disease. There is no cure once it takes hold, so the defense is all prevention: choose resistant varieties, space plants for good airflow, and water at the base so the leaves stay dry. Avoid overhead watering, especially late in the day.

Don't over-feed

Heavy fertilizing pushes lush growth at the expense of the aromatic oils that give basil its flavor. Keep feeding modest — the goal is taste, not size.

Common pests

  • Japanese beetles
  • Aphids
  • Slugs
  • Spider mites
  • Whiteflies
  • Thrips

Common diseases

  • Downy mildew
  • Fusarium wilt
  • Gray mold (Botrytis)
  • Bacterial leaf spot
  • Root rots (Pythium/Rhizoctonia)

Sources